i have a 2006 mustang (4.0 RIP) with magnaflow catback exhaust and bbk headers. car sounds AMAZING for a 4.0 6cyl... until ive been driving for a few hours. it doesnt sound BAD per se. but the exhaust sound seems to dramatically fade away after everythings gotten warm and what not. its back to normal after i let it sit for a long while or seemingly when i add fuel to the tank. any clues, ideas, answers as to why it would be doing this?!

 

Pretty sure they all do this.

My GT does. I'm sure it has something to do with the material in the muffler heating up. Either the packing or the steel casing.

 

My GT does this also, no worries.

 

Ditto